This invention relates to pressure detecting apparatuses, and more particularly to an improved pressure detecting apparatus in which the linearity of the response characteristic is improved and temperature compensation is effected for the output voltage variation.
A pressure detecting apparatus is known in the art for measuring a fluid pressure comprising a bridge circuit having semiconductor pressure sensitive elements adapted to convert a pressure variation into an electrical signal on the basis of the piezo resistance effect and a reference resistor providing a constant resistance independent of temperature variation. A constant voltage is applied to the bridge circuit and the output of the bridge circuit is amplified and converted into a current or voltage signal, whereby an electrical signal corresponding to the pressure variation is obtained.
The pressure-resistance variation characteristic of the semiconductor pressure sensitive element itself is non-linear. Therefore, even if such a bridge circuit is formed to detect the pressure, the pressure-voltage characteristic, or the response characteristic, of the bridge circuit is non-linear.
This nonlinear response characteristic is due to the error of the pressure sensitive element. For instance, in the case where a measuring instrument such as a recorder having a linear type meter is connected at the rear stage, measurement errors are caused.
Accordingly in order that the pressure detection should not be affected by the error of the pressure sensitive element, only a part of the output characteristic of the pressure sensitive element, which is satisfactory in linearity, is employed, or the output of the bridge circuit is electrically corrected after being amplified. This is the conventional method of minimizing the effect of the error of the pressure sensitive element.
However, in the first means of the conventional method, only a linear part of the output characteristic is utilized as was described, and therefore the first means is disadvantageous in that the pressure detection range is reduced. On the other hand, in the second means the nonlinearity can be corrected by signal process only in a predetermined pressure detection range; however, the second means is also disadvantageous in that in a diffrent pressure detection range the output variation width for the pressure detection range fluctuates thereby causing error even with the one and same pressure width. For this reason, in detecting a pressure in a different pressure range it is necessary to change the aforementioned electrically correcting circuit constant employed after amplification, which results in inconvenience in pressure detection.
Furthermore, adjustment of the linearity of the pressure-electricity characteristic is made at several points in a predetermined pressure detection range. Therefore, it is difficult to maintain the linearity over the entire predetermined pressure detection range. In addition, maintaining the linearity is more difficult when the pressure detection range is changed.
The pressure widths corresponding to certain output variation widths are not always coincident with one another. As a result, adjustment of the input to a measuring instrument connected at the rear stage becomes rather intricate, which leads to errors in measurement.
The output characteristic of the bridge circuit causes a pressure detection error depending on temperature variations because the semiconductor pressure sensitive element has a resistance-temperature coefficient. In order to overcome this difficulty, a thermistor having a negative resistance-temperature coefficient is inserted between the bridge circuit and the constant voltage source, so that the input voltage to the bridge circuit is changed in association with the temperature variation, to carry out thereby the temperature compensation.
However, in this conventional method, it is difficult to vary the input voltage in coincidence with the temperature-resistance characteristic of the bridge circuit, and the temperature compensation by the thermistor is applicable only to temperature variation in a very small range.
It may be considered to drive the bridge circuit by a constant current source instead of the constant voltage source. However, in this case a rather intricate circuit is required for the compensation of the nonlinearity. Accordingly, it is difficult to put such a circuit in practical use in view of stability and reliability.